Cobalt Complexes As Single Redox Molecules for Catholyte and Anolyte in Redox Batteries

Wednesday, 4 October 2017: 17:20
Chesapeake K (Gaylord National Resort and Convention Center)
G. Nikiforidis (KAIST), C. Yang, Y. Luo, Z. Hou (RIKEN), and H. R. Byon (KAIST)
We herein present the design and realization of chelating ligands in modulating redox potentials of the central metal ion, offering a single redox molecule to both catholyte and anolyte applicable in the field of redox batteries. We demonstrated an increase in redox potentials from the bidentate and tridentate ligands that are coordinated with Co(II) forming pseudo-octahedral complexes. The cyclic voltammetry (CV) curves using a three-electrode cell show quasi-reversible behavior of the Co(I/II) and Co(II/III) redox couples in non-aqueous electrolyte solutions such as carbonate and glymes. Further, the potential of Co(II/III) is altered due to the electron deficiency of ligands. Amongst the ligands investigated, tridentate and the most electron deficient one provide the highest peak to peak voltage, equal to ~1.85 V. The single species of Co complex in the redox cells with 200 mM delivers ~22.3 mAh L-1 and ~98% capacity retention for 300 cycles at a current density of 0.2 mA cm-2 and 35ºC (Fig.1), yielding a volumetric energy density of 40.7 Wh L-1. At a current density of 0.5 mA cm-2 Coulombic efficiency lingers at ~96% over 600 cycles, demonstrating a promising redox cell with long-term cycling stability. The ligand design associated redox-reaction process and the stability of Co complex for galvanostatic cycling would be further discussed in the presentation.